when I run this command:
Get-AzureADMSPrivilegedRoleDefinition -ProviderId aadRoles -ResourceId $global:varTenant.ObjectId | Format-List
None of the results have any values for the fields I've outlined in red. For example, roles that I know have active assignments don't show any value in the ActiveAssignmentCount property. How can I get those values to populate?

I tried in my environment and got below results:
When I execute the commands and got same output like below :
PIM.You can get the active and eligible assignments by running this command:
Output:
Make use of below scripts to get the Active assignments count and Eligible assignments count.
Activeassignmentscount:
To get the specific user for
Activeassignmentcountyou can use this script.Powershell:
Eligibleassignmentscount:
To get the specific user for
Eligibleassignmentcountyou can use this script.Refer this link you can also get the process through
graph explorer.